In the commercial production of paper the raw paper is typically wound on a tubular core. Most cores used for winding raw paper are made entirely of steel or entirely of multi-layer laminated paper. Although steel cores are reusable, they are heavy and difficult to handle. They are also expensive. Paper cores, on the other hand, have considerable advantages in that they are light, easy to use, relatively inexpensive compared to steel cores, and may be recycled. However, they tend to be dimensionally unstable and therefore not reusable with any degree of reliability or predictability.
Paper cores are generally manufactured according to a well known process wherein a number of strips of fibrous material, typically Kraft paper of various grades, are spirally wound on a rotating mandrel in overlying relationship to one another. Multiple plies of the paper are wound over one another to increase the thickness of the wall of the core. The thickness of the wall of the core will depend upon the particular end use for the core; where a stronger core is required the wall thickness is increased accordingly. An adhesive is generally applied between the plies as they are spirally wound to increase the strength of the resulting tube. Spiral winding is a continuous process; the tube is cut to the desired length as it is formed.
Paper cores may also be formed in a batch type operation wherein a single, wide sheet of fibrous material such as Kraft paper is wound onto a rotating mandrel until the desired number of plies and thickness have been achieved. This process is slower than the spiral winding process since only one tube may be formed at a time, and is therefore used much less often.
Whether a core for holding raw paper is manufactured by spiral winding or by single sheet winding, end caps known as tips are often attached to each end of the core to facilitate attachment of the core to the paper winding and processing machinery. The tips also protect the ends of the core from mechanical damage.
Unlike metal cores for paper rolls, which may be used repeatedly, most paper cores are limited in the number of times that they may be used. Often they may be used only once because of their dimensional instability. As ambient humidity changes, the paper in the core tends to absorb moisture from, or lose moisture to the atmosphere. This causes dimensional changes to occur, often resulting in the core exceeding the allowed tolerances in dimensional specifications. When the dimensional tolerances are exceeded the core must be discarded or recycled since it can no longer be used on the paper machines.
Because prior paper cores for supporting rolls of raw paper are known to be vulnerable to dimensional changes with changes in ambient humidity, each core must be carefully inspected and measured before it can be reused. Such inspection is labor intensive and time consuming, and therefore expensive. As a result, paper cores are often simply discarded after a single use. Thus, a need exists for a reusable core for supporting paper rolls that has all of the advantages of a core made entirely of paper and yet is dimensionally stable so that it can be reused. This is the primary objective of the present invention.
Core tubes made of composite materials, including paper, for purposes other than supporting rolls of paper, are known. Such tubes may or may not be dimensionally stable, depending on their construction and the environments in which they are used. In any case, in the applications for such composite tubes, dimensional stability is apparently not of concern. Accordingly, such prior composite core tubes do not suggest an acceptable solution to the problem of providing a dimensionally stable and therefore reusable core tube made in a largely conventional manner primarily of paper and suitable for supporting rolls of paper. For example, in U.S. Pat. No. 2,755,821 to Stahl, a spirally wound tube for use as a core in electrical applications is disclosed. In that patent a tube is provided having an inner strip of insulating paper overlaid by a central layer of a dielectric material such as thermo-setting polyester film. The dielectric layer is overlaid by an outer insulating layer, typically paper. This tube is useful for electrical applications such as a base for coil and transformer windings. U.S. Pat. No. 3,451,433 to Cunningham discloses a spirally wound core tube having a combination of multiple plies of resin impregnated textile material and at least one ply of paper bonded to the textile plies. This core is used as a forming tube for spun glass filaments. Similarly, U.S. Pat. No. 3,587,656, to Cunningham is directed to a multi-layer, spirally wound tube for use as a forming tube for spun glass fibers, in which all the layers are of paper, but in which the outer and end surfaces are impregnated with a synthetic resin. This results in a tube with a hard outer layer and end surface of resin impregnated paper. The resin increases the strength and moisture resistance of the outer and end surfaces of the tube, yet provides a smooth peripheral surface upon which the fibers are spun.